[swift-users] UnsafeMutableRawPointer to UnsafeMutablePointer<T>: EXC_BAD_ACCESS on pointee

[Michael Ilseman]

> On Sep 23, 2017, at 3:44 AM, nyg nyg via swift-users <swift-users at swift.org> wrote:
> 
> Hello all,
> 
> I'm trying to get an UnsafeMutablePointer from an
> UnsafeMutableRawPointer obtained using the Unmanaged structure:
> 
> class C { var foo = 42, bar = "bar" }
> let c = C()
> 
> let rawPointer = Unmanaged.passUnretained(c).toOpaque()
> 

I believe that the object “c” is effectively dead from this point onwards. Did you try putting a use of c later on to guarantee its lifetime? E.g. add a `dump(c)` at the bottom of your script?

> let pointer = rawPointer.bindMemory(to: C.self, capacity: 1)
> let pointee = pointer.pointee
> print(pointee.foo) // EXC_BAD_ACCESS
> 
> Here's some LLDB output, which looks strange to me as everything seems
> alright in pointer until I ask for its pointee:
> 
> (lldb) frame variable -L c
> scalar: (memtest2.C) c = 0x0000000101d00030 {
> 0x0000000101d00040: foo = 42
> 0x0000000101d00048: bar = "bar"
> }
> (lldb) frame variable -L rawPointer
> 0x00000001005e2e08: (UnsafeMutableRawPointer) rawPointer = {
> scalar: _rawValue = 0x0000000101d00030 {
> 0x0000000101d00040: foo = 42
> 0x0000000101d00048: bar = "bar"
> }
> }
> (lldb) frame variable -L pointer
> 0x00000001005e2e10: (UnsafeMutablePointer<memtest2.C>)
>                                            pointer = 0x0000000101d00030
> (lldb) frame variable -L pointer._rawValue
> scalar: (memtest2.C) pointer._rawValue = 0x0000000101d00030 {
> 0x0000000101d00040: foo = 42
> 0x0000000101d00048: bar = "bar"
> }
> (lldb) frame variable -L pointee
> 0x00000001005e2e18: (memtest2.C) pointee = 0x00000001005b65d8 {
> 0x00000001005b65e8: foo = 140736790071664
> 0x00000001005b65f0: bar = ""
> }
> 
> I've also tried assumingMemoryBound(to:) or simply doing:
> 
> let pointer = UnsafePointer<C>(bitPattern: Int(bitPattern: rawPointer))!
> print(pointer.pointee.foo) // EXC_BAD_ACCESS
> 
> But I always get this EXC_BAD_ACCESS error. What is going on here?
> 
> 
> Thanks for your help,
> Nick
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